This invention relates to a negative glow discharge lamp of small size, and more particularly to a lamp structure which is suitable for preventing undesirable degradation of lamp operation characteristics due to occurrence of graphitization while the lamp is in operation.
A low voltage electric discharge lamp of small size having at least one pair of electrodes and discharging gas enclosed in a tightly formed discharge container is disclosed in JP-A No. 63-19750. The disclosed discharge lamp employs an electrode arrangement in which, when one of the electrodes acts as an anode, the electrode acting as the anode is located in a negative glow domain so as to ensure zero anode drop voltage. Through the employment of the electrode arrangement described above, both the lamp voltage and the discharge voltage of the low voltage electric discharge lamp of small size can be lowered to increase the luminous efficiency of the discharge lamp.
For such a type discharge lamp, U.S. Pat. No. 4,879,493 may be referred to which is entitled "Low-pressure discharge lamp" to Matsuno et al and assigned to Hitachi, Ltd. The inventors of the present invention disclose improvements for stabilizing electrical characteristics and broadening frequency range of such a discharge lamp in U.S. Pat. application No. 321,069, filed Mar. 9, 1989 entitled "Negative glow discharge lamp device" and assigned to Hitachi, Ltd.
The size of the discharge lamp described above is sufficiently small when used as an ordinary fluorescent lamp. However, the outer diameter of the glass tube forming the discharge container is relatively large, and it may be frequently impossible to insert the discharge lamp into, for example, a very narrow space in which the discharge lamp is to be installed. Further, the foregoing type discharge lamp is still insufficient in its operation characteristics, especially, in the aspect of preventing undesirable graphitization of the glass tube due to scattering of thermion radiation matter.